The present invention generally relates to a sound reinforcement system and more particularly, to a howling canceller for cancelling howling of the sound reinforcement system by using a digital filter.
Due to recent development of conference systems based on telecommunication, there is a keen demand for a howling canceller which enables a sound reinforcement system to louden sounds clearly and naturally.
Hereinbelow, a known howling canceller provided with a digital filter will be described with reference to FIG. 1. The known howling canceller includes a microphone 1, a microphone amplifier 2, a subtracter 3, a transfer characteristic estimation unit 4 for storing therein a transfer characteristic between the microphone 1 and a loudspeaker 8 as an impulse response therebetween, and a switch 5 for controlling operations of the estimation unit 4. When the switch 5 is closed, the transfer characteristic is estimated by the estimation unit 4. On the contrary, when the switch 5 is opened, estimation of the transfer characteristic by the estimation unit 4 is stopped.
The known howling canceller further includes a digital filter (hereinbelow referred to as an "echo signal estimation unit") 6, a loudspeaker amplifier 7, a switch 9 and a source 10 of white noises. The echo signal estimation unit 6 performs convolution of the impulse response stored in the transfer characteristic estimation unit 4 and an input signal applied to the loudspeaker amplifier 7. Meanwhile, the switch 9 is arranged to disconnect the microphone 1 from the loudspeaker system including the loudspeaker amplifier 7 and the loudspeaker 8 and input the white noises to the loudspeaker system when the transfer characteristic is estimated by the estimation unit 4.
Hereinbelow, operations of the known howling canceller of the above described arrangement will be described. Initially, the switch 5 is closed and the switch 9 is connected to a contact B such that the transfer characteristic, i.e., the impulse response between the loudspeaker 8 and the microphone 1 is estimated by the estimation unit 4. The white noises outputted from the source 10 are loudened by the loudspeaker amplifier 7 and the loudspeaker 8 and then, are received by the microphone 1. The white noises outputted from the source 10 are also inputted to the echo signal estimation unit 6. In the echo signal estimation unit 6, convolution of the signal of the white noises and the estimated impluse response delivered from the estimation unit 4 is calculated so as to obtain an estimated echo signal. Then, in the subtracter 3, the estimated echo signal of the estimation unit 6 is subtracted from an output of the microphone amplifier 2. Thereafter, the output of the subtracter 3 is transmitted to the transfer characteristic estimation unit 4 which corrects the estimated impulse response stored therein so as to reduce the output of the subtracter 3. Subsequently, by using the corrected estimated impulse response, calculation and subtraction of the estimated echo signal are again performed in the estimation unit 6 and the subtracter 3, respectively, so that a new output is delivered from the subtracter 3 and thus, the estimated impulse response is again corrected by the estimation unit 4 so as to reduce the new output of the subtracter 3. When the above described operational procedure has been repeated a sufficient number of times, the output of the subtracter 3 finally becomes zero. As a result, an impulse response subjected to excellent approximation is stored in the estimation unit 4. Thus, after the transfer characteristic has been estimated by the estimation unit 4, the switch 5 is opened so as to prevent the estimation unit 4 from correcting the estimated impulse response. Then, the switch 9 is connected to a contact A such that sounds inputted to the microphone 1 are loudened by the loudspeaker amplifier 7 and the loudspeaker 8. At the time of loudening of the sounds inputted to the microphone 1, since an echo signal delivered from the loudspeaker 8 to the microphone 1 is estimated from the output of the microphone 1, the howling loop is interrupted and thus, generation of howling is cancelled.
Arithmetic operations of the transfer characteristic estimation unit 4 and the echo signal estimation unit 6 are performed digitally. Supposing that an input signal applied not only to the estimation unit 6 but to the loudspeaker amplifier 7 and an estimated impulse response inputted from the estimation unit 4 to the estimation unit 6 are, respectively, represented by x(n) and h(n), an estimated echo signal y is expressed by the following equation (1) for calculating the convolution: ##EQU1## where:
N=total number (hereinbelow referred to as a "tap number") of digitized sampling data of analog signals x and h, and
n=serial number of the tap number N.
Assuming that a character .DELTA.t denotes a sampling period for digitizing the analog signals x and h, a time period T of the estimated impulse response h and a high frequency limit F are given by the following equation (2): ##EQU2##
Accordingly, in the case where the sampling period .DELTA.t is constant, it becomes possible to eliminate longer echoes as the tap number N is increased. On the other hand, as the sampling period .DELTA.t is reduced, it becomes possible to eliminate echoes of higher frequency.
However, in the above described arrangement of the prior art howling canceller, since echoes of signals up to a high frequency range are required to be eliminated, the sampling period .DELTA.t is reduced. Thus, in the case where it is impossible to increase the tap number N to a large value, an echoing time period to be eliminated is reduced and thus, howling cannot be cancelled sufficiently.
Furthermore, since increase of the tap number N leads to increase of a time period required for calculating the equation (1), such problems arise that it becomes impossible to perform the real-time processing and production cost of the prior art howling canceller rises extremely.
Moreover, in the above described arrangement of the known howling canceller, if the white noises emitted from the loudspeaker 8 at the time of estimation of the transfer characteristic, i.e., a level of an estimated sound of the transfer characteristic is not raised considerably, a satisfactory S/N ratio cannot be obtained in a room of poor sound proof, thereby resulting in unsatisfactory cancellation of the howling. In addition, in the room of poor sound proof, such problems are encountered that noises outside the room penetrate into the room and sounds inside the room leak out of the room.